Conveyor belts for transporting tar sands

ABSTRACT

A method for transporting tar sands in an open pit mine utilizing novel flexible belt conveyors between a receiving area and a discharge area which comprises providing a conveyor belt having an upper surface layer of a novel elastomeric material which is flexible at low temperatures and is substantially resistant to excess swelling when exposed to petroleum liquids containing up to 30 volume percent aromatics at temperatures in the range of -60°F to +100°F, applying a thin layer of said petroleum liquid to the surface of said belt and thereafter transferring tar sands on said belt from a receiving area to a discharge area.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of copending application Ser. No. 509,205filed Sept. 25, 1974.

BACKGROUND OF THE INVENTION

Large quantities of tar sands can be found in relatively shallowdeposits in various localities throughout the world. The largest ofthese deposits is found in the Canadian province of Alberta. Thesedeposits which are commonly referred to as Athabasca tar sands containupwards of 350 billion barrels of recoverable bitumen and underlie morethan 3000 square miles at depths of 0 to 2000 feet. A large part ofthese tar sands can be recovered by open pit strip mining. The provinceof Alberta is well known for its long harsh bitter cold winters and itsshort but often hot summers. Temperatures over a twelve-month periodranging from a low of -50°F to a high of 95°F are not unusual innorthern Alberta. Mining tar sands under these conditions presents someunique problems of digging and transportation heretofore notencountered.

Bituminous tar sands such as the Athabasca tar sands comprise asiliceous material, generally having a size greater than that passing a325 mesh screen, saturated with a relatively heavy, viscous bitumen inquantities of from 5 to 21 weight percent of the total composition. Moretypically, the bitumen content of the sands is about 8 to 15 percent.This bitumen is quite viscous and contains typically 4.5 percent sulfurand 38 percent aromatics. Its specific gravity at 60°F ranges typicallyfrom about 1.00 to about 1.06. The tar sands also contain clay and siltin quantities of from 1 to 50 weight percent of the total composition.Silt is normally defined as mineral which will pass a 325 mesh screenbut which is larger than 2 microns. Clay is mineral smaller than 2microns including some siliceous material of that size.

There are several well-known processes for effecting separation ofbitumen from the tar sands. In the so-called "cold water" method, theseparation is accomplished by mixing the sands with a solvent capable ofdissolving the bitumen constituent. The mixture is then introduced intoa large volume of water, water with a surface agent added, or a solutionof a neutral salt in water. The combined mass is then subjected to apressure or gravity separation.

In the hot water method, the bituminous sands are jetted with steam andmulled with a minor amount of hot water at temperatures in the range of140°F to 210°F. The resulting pulp is dropped into a stream ofcirculating hot water and carried to a separation cell maintained at atemperature of about 150°F to 200°F and usually 185°F. In the separationcell, sand settles to the bottom as tailings and bitumen rises to thetop in the form of an oil froth. An aqueous middlings layer containingsome mineral and bitumen is formed between these layers. A scavengerstep may be conducted on the middlings layer from the primary separationstep to recover additional amounts of bitumen therefrom. This stepusually comprises aerating the middlings as taught by K. A. Clark, "TheHot Water Washing Method", Canadian Oil and Gas Industries 3, 46 (1950).These froths can be combined, diluted with naphtha, and centrifuged toremove more water and residual mineral. The naphtha is then distilledoff and the bitumen is coked to a high quality crude suitable forfurther processing. The hot water process is described in detail byFloyd et al. in Canadian Pat. No. 841,581 issued May 12, 1970.

The tar sands must be mined from their deposits for charge into theparticular process used to effect separation of the bitumen from thesands. In one particular mining operation, overburden is first removedfrom he deposits. This involves the stripping away of trees, muskeg, andearth which overlie the tar sands. The sands are then mined by giantbucketwheel excavators. The digging wheels on such excavators can haveten or more buckets each bucket capable of biting out and holding nearlytwo tons of sand. Sand dug by the wheel travels the length of the diggerby conveyor to the discharge boom and is dropped on the first of severalconveyor belts.

One particular operation utilizes a first conveyor which is 4,600 feetlong, 60 inches wide, and made of one-inch thick rubber which is avulcanized mixture of about 15 percent natural rubber and about 85percent styrene-butadiene copolymer rubber on steel cords. It travels at1,050 feet per minute, or about 12 miles per hour. The sand is dumpedfrom the first conveyor onto a second which is 60 inches wide and about1000 feet long. As diggers get farther from the initial mining area, thesecond conveyor is extended and ultimately both conveyors stretchseveral miles. The second conveyor drops sand onto a third conveyor.This third belt, 72 inches wide, runs 1,350 feet to the separationplant. This system has been used commercially at Fort McMurray, Alberta,for a number of years.

Although belt conveyors have been widely used in other miningoperations, there are some new problems peculiar to the handling of tarsands. Up until now conveyor belts were made from natural rubber orstyrene-butadiene copolymer or mixtures of the two. These compounds areselected because they retain good flexibility over a wide temperaturerange, e.g., -60°F to 150°F. However, when these belts are used in a tarsands mining operation, it has been observed that large quantities ofthe sands stick to the belts causing fouling and a decrease in beltcapacity.

Depending on the character of the feed material and the outsidetemperature, the layer of sticking sands attains a thickness of as muchas one-half inch. In warm weather the belt deposit increases intoughness and thickness. In cold weather, the deposit becomes frozen intransit from the mining area to the processing area. Steam jets andscrapers have been proposed for the purpose of removing the deposit butnone of these means has been fully satisfactory. Accumulation of sandson the conveyor belts causes unbalanced loads, straining problems, andadditional wear on pulleys, idlers, and scrapers.

Canadian Pat. No. 922,655 issued Mar. 13, 1973 to William Hogg disclosesan improvement to a process for transporting bituminous tar sands on aconveyor belt between a receiving area and a discharge area whichgenerally comprises applying a layer of an aqueous liquid medium to thesurface of the conveyor prior to transporting tar sands on the beltsurface. The aqueous layer reduces the tendency of the tar sands tostick to the belt when discharged at the discharge area. When operatingthis process at temperatures above freezing, it has been found to bereasonably effective. However, when the temperature falls belowfreezing, it is necessary to add freezing point depressants such asalcohols to keep the water from freezing on the belt surface. Ethyleneglycol is one of the preferred additives because of the safety of itsuse as well as its compatibility with subsequent tar sand processingsteps. However, the relatively high expense of ethylene glycol,particularly in colder climates, reduces its attractiveness as an answerto the conveyor belt cleaning problem. Thus, although the disclosure ofthe above Canadian patent provides one technically feasible means ofreducing the problems of transporting tar sands on a conveyor beltsystem, substantial room for improvement still exists.

One method employed to overcome the problem of tar sands sticking to thesurface of natural rubber or styrenebutadiene rubber conveyor belts hasbeen the addition of kerosene to the surface of the belt before the tarsands are placed thereon. During cold weather operation, that is atambient temperatures below 10°F, this method of keeping conveyor beltsreasonably clean has been generally acceptable. However, when thetemperature rises above 10°F the natural rubber or styrene-butadienerubber will absorb the aromatic hydrocarbon liquids of the kerosene andbegin to swell. The aromatic swollen belt would often expand to thedegree that it becomes inoperable and the belt must be shut down.

One approach to overcome the swelling of the conveyor belt caused byapplication of liquid hydrocarbons is to select a rubber composition forthe belt surface which is resistant to swelling. A rubber well-known forits resistance to petroleum liquids is nitrile rubber.

In The Vanderbilt Rubber Handbook edited by G. G. Winspear (R. T.Vanderbilt, Inc., New York, 1968) pages 99 to 118, methods ofpreparation as well as physical and chemical characteristics of nitrilerubber are disclosed. It is disclosed in this text that the oil andchemical resistance of nitrile rubber is the major factor dictating itsuse. The oil resistance of compounds based on nitrile rubber isdetermined by the acrylonitrile content of the nitrile rubber. Also, itis disclosed that the low temperature properties of nitrile rubbercompounds vary with the acrylonitrile content of the polymer and thetype of plasticizer incorporated therein.

It is also disclosed that nitrile rubber has a wide range ofcompatibility with other polymers, making possible a great variety ofunique and desirable properties. Blends can be made with vinyl chloride,phenolic and ABS resins, SBR, polychloroprene, cis polybutadiene,chlorosulfated polyethylene, thiokol, and to a certain extent, naturalrubber. All of these blends are used to impart variations in processing,ozone resistance, low temperature flexibility and cost. The use ofnitrile rubber for conveyor belts is disclosed. Yet with all thisinformation available, the industry has up until this point been unableto provide a flexible rubber conveyor belt suitable to transport tarsands in an open pit mine without having to shut the belt down becauseof tar sands sticking to the belt surface.

By the method of the present invention using the novel elastomercompositions herein provided, tar sands can be efficiently andeconomically transported in an open pit mine under a wide range ofweather and temperature conditions.

DESCRIPTION OF THE INVENTION

Transportation of tar sands on rubber conveyor belts in an open pit mineis a process which has only recently been tested on a commercial scale.Rubber conveyor belts which have the physical characteristics necessaryto perform satisfactorily in this procedure have not heretofore beenavailable. The present invention provides novel conveyor belts of novelrubber compositions which meet these requirements.

To operate twelve months a year in a climate such as that which existsin northern Alberta, Canada, a conveyor belt must be maintainedsubstantially free of tar sand build up and be flexible at temperaturesbetween +100°F and -100°F.

It has been determined that a conveyor belt composition which has avolume percent increase of 40 or less according to the procedures ofASTM D-471 when immersed in number 3 ASTM oil for 24 hours will notswell to the point that it is inoperable at temperatures up to 100°Fwhen coated during normal use to prevent tar sand buildup with keroseneor any liquid hydrocarbon mixture containing up to 30 volume percentaromatics.

The present invention provides a flexible continuous conveyor belthaving a novel rubber surface which can be exposed to liquidhydrocarbons containing up to 30 weight percent aromatics over a widerange of temperatures without causing the belt to become inoperablebecause of excessive swelling of the rubber in the belt. At the sametime the rubber in this conveyor has sufficient low temperatureflexibility to permit the conveyor to operate at temperatures down to-100°F.

The present invention provides novel rubber compositions particularlysuited for use in rubber conveyor belts used to transport tar sands. Thepresent invention also provides a method for transporting tar sands inan open pit mine subject to a wide range of temperatures in a mannerwhereby adherence of sticky tar sands to the belt surface issubstantially reduced.

Normally in transporting tar sands or any mineral ore, the conveyor beltis supported on idler rollers or equivalent support means so that it isshaped as a long trough. The belt must have good abrasion resistance aswell as good tensile properties. Most importantly the belt must beflexible over a wide temperature range.

The essential element in providing a process for transporting tar sandsas herein disclosed is the composition on the surface of the conveyorwhich comes in contact with tar sands. This composition, if rubber, mustmeet the critical requirements of controlled swell when contacted witharomatic liquid hydrocarbons as well as low temperature flexibilitywhich is critical to operating a conveyor belt in cold northernclimates. As discussed above, none of the prior art elastomercompositions have been found which meet these critical requirements.

It has been discovered that specific blends of nitrile rubber andcertain other liquid and solid plasticizers prepared to form a conveyorbelt as herein disclosed provides a conveyor system which when coatedwith liquid petroleum hydrocarbons containing up to thirty volumepercent aromatics is effective in transporting tar sands in an open pitmine under a wide range of temperature conditions. By this method theconveyor belt remains substantially free off excessive tar sandsadhering thereto.

Specifically an improved rubber composition suitable for use over a widerange of temperatures and resistant to excess swelling when exposed toaromatic-containing liquid hydrocarbons has been discovered. This rubbercomposition comprises a vulcanized mixture of:

a. 100 parts of a rubber mixture comprised of 20 to 95 weight percentnitrile rubber and 80 to 5 weight percent of a second rubber selectedfrom the group consisting of polybutadiene, styrene-butadiene copolymer,neoprene, polyisoprene and natural rubber said rubber mixture having anacrylonitrile content in the range of 5 to 35 weight percent and

b. 1 to 30 parts by weight of a liquid plasticizing agent compatiblewith said mixture, said vulcanized mixture being characterized as havinga volume percent swell after 24 hours in ASTM number 3 oil at 158°F inthe range of 1 to 40 as measured according to ASTM D-471 and a GehmanT₁₀₀ stiffness value in the range of -30°F to -100°F as measuredaccording to ASTM D-1053.

The novel vulcanized rubber compositions of the present invention areprepared as a blend of nitrile rubber, a second rubber selected from thegroup given above which serves as a solid plasticizing agent for thenitrile rubber and a liquid plasticizing agent which is compatible withthe mixture.

Nitrile rubber is prepared by copolymerizing acrylonitrile and butadienein accordance with procedures such as those outlined on pages 99 to 117of the Vanderbilt Rubber Handbook noted above. The copolymer can beprepared so that the desired level of acrylonitrile is present toprovide the necessary weight percent of acrylonitrile in the finalrubber mixture. On a commercial basis nitrile rubber compositionscontaining 20, 28, 33, 40, and 50 weight percent acrylonitrile areavailable. The nitrile rubber is blended by any of the means well knownin the art with the second rubber additive of the composition to providea rubber mixture containing an acrylonitrile content in the desired 5 to35 weight percent range based on the weight of the rubber mixture.

A liquid plasticizing agent which is compatible with the rubber is alsoblended with the mixture. This mixture is thereafter blended with knownvulcanizing additives and vulcanized to provide the novel rubbercompositions suitable for use in the applications herein disclosed.

Nitrile rubber, although highly resistant to swell in the presence ofliquid hydrocarbons, becomes stiff and inflexible at low temperatures. Aliquid plasticizing agent can be added to the nitrile rubber to improveits low temperature flexibility. However, liquid hydrocarbons and inparticular aromatic-containing liquid hydrocarbons are known to extractthe liquid plasticizer from the nitrile rubber thereby negating theflexibility improvement achieved by the addition of the plasticizer. Theextraction of the plasticizer from a nitrile rubber is observed in oneway by negative swelling which sometimes is measured by the proceduresof ASTM D-741 using ASTM number 3 oil for 24 hours at 158°F. A nitrilerubber composition containing a plasticizing agent which is found tohave a negative volume swell as measured according to the aboveprocedures generally does not have the low temperature flexibility torender it useful in many applications and in particular for use inconveyor belts transporting tar sands. Data regarding the effects ofASTM number 3 oil on plasticized nitrile rubber can be found in anarticle "Tech Book Facts CHEMIGUM COMPOUNDING CC-42" published by theGoodyear Chemicals Division of The Goodyear Tire and Rubber Company ofAkron, Ohio.

Now, it has been discovered that by adding a second rubber whichfunctions as a solid plasticizing agent to the nitrile rubber and theliquid plasticizing agent, a rubber mixture can be provided which whenvulcanized has the necessary physical characteristics to meet therequirements of the desired uses, in particular the use of transportingtar sands in an open pit mine.

The second rubber in the mixture which serves as a solid plasticizingagent for the compositions of the present invention can be selected fromthe group consisting of polybutadiene, styrene-butadiene copolymer,neoprene, polyisoprene and natural rubber. Each of these elastomers arewell known to those skilled in the art.

The mixtures of the nitrile rubber and the second rubber should becomprised of 20 to 95 weight percent nitrile rubber and 80 to 5 weightpercent of the second rubber. The preferred composition of the mixtureis 40 to 90 weight percent nitrile rubber and 60 to 10 weight percent ofthe second rubber. In all compositions the acrylonitrile content must bein the range of 5 to 35 weight percent based on the weight of themixture. The second rubber component in the mixture can be any of thedisclosed second rubber additives or mixtures thereof. For example, thesecond rubber additive can contain 50 weight percent polybutadiene and50 weight percent styrene-butadiene copolymer.

A wide variety of suitable plasticizers which are compatible withnitrile rubber and the second rubber, are well known to those skilled inthe art. These plasticizers in general include esters of bibasic acidsas well as synthetic liquid polymers. As those skilled in the art know,a plasticizer is considered compatible if there is no significantmigration of the plasticizer from the rubber compound after storage atroom temperature for a period of four to six weeks after the mixture hasbeen prepared. On occasion slight migration of a compatible plasticizerto the surface of the rubber compound may occur, but unless there issubstantial migration, the plasticizer is considered compatible.Included among those plasticizers but not limited thereto are trioctylphosphate, dibutyl sebacate, diisooctyl sebacate, tributyl citrate,di(butoxy-ethoxy-ethyl) formal, di(butoxy-ethoxy-ethyl) adipate, dioctylsebacate, dioctyl phthalate, dibutyl phthalate and tributoxy oxyethylphosphate. The liquid plasticizer can be added to the rubber mixture inquantities of 1 to 30 parts by weight per 100 parts of the rubbermixture. The preferred quantity of liquid plasticizer added to therubber mixture is 20 to 30 parts plasticizer per 100 parts of rubber.The preferred liquid plasticizers are di(butoxy-ethoxy-ethyl) formal anddi(butoxy-ethoxy-ethyl)adipate.

The rubber compositions can also contain reinforcing agents where theuse requires. Reinforcing agents suitable for use include carbon black,hard clays and hydrated silica with carbon black being preferred. Thereinforcing agent can be added to the rubber mixture in quantities of 30to 60 parts reinforcing agent per 100 parts by weight of rubber mixture,with 45 to 55 parts of reinforcing agent per 100 parts of rubber mixturebeing preferred. The reinforcing agent is added primarily to improveproperties other than low temperature flexibility and volume percentswell in liquid hydrocarbons. Normally the effect on these properties isminimal. Accordingly, although the reacted swell and stiffnessproperties apply to the reinforced composition when a reinforcing agentis employed, the unreinforced composition will also have the recitedswell and stiffness properties. Standard well known vulcanizing agentsare also added to the rubber composition by methods, well known in theart and the rubber compound can be vulcanized by standard procedureswell known in the art for vulcanizing nitrile rubber.

As one means of further defining one mode of the compositions of thepresent invention a series of six rubber compositions were preparedusing nitrile rubber, a second rubber and a liquid plasticizercompatible with nitrile rubber. The compositions and their respectivecomponents are listed respectively as examples 1 through 6 in the tableherein provided.

Examples 1 through 5 disclose compositions containing varying quantitiesof acrylonitrile in the rubber according to the invention hereinprovided. Example 6 is a composition having a quantity of acrylonitrilein the rubber mixture which is outside the limits provided for thecompositions of this invention. As is dislosed, the physical propertiesof the rubber composition of Example 6 fail to meet the necessarylimitations of the compositions of this invention. Thus the compositionof Example 6 not meeting either the volume percent swell or lowtemperature flexibility limits falls outside the scope of the presentinvention. The rubber compositions given in the table were blended on astandard rubber mill and cured for 30 minutes at 295°F.

Although the compositions given in the table contain styrene-butadienecopolymer, any of the other second rubber additives selected frompolybutadiene, polyisoprene, neoprene and natural rubber and mixturesthereof can be substituted for the styrene-butadiene copolymer withequivalent results being obtained. Also, other rubber compatible liquidplasticizing agents as well as other suitable reinforcing agents can besubstituted with equivalent results being obtained.

Compositions of the present invention are particularly suitable for usein flexible rubber conveyor belts. In particular they are useful inconveyor belts which are normally coated with liquid hydrocarbons suchas kerosene when transporting tar sands. Thus the present inventionincludes a flexible conveyor belt particularly suited for use intransporting tar sands which belt has a limited resistance to swellingwhen exposed to liquid hydrocarbons containing up to 30 weight percentaromatics. The conveyor belt comprises a rubber belt having a pluralityof metal cables longitudinally disposed in said belt and arrangedsubstantially parallel to each other. The rubber component of the belt,or at least the surface thereof, preferably comprises a vulcanizedmixture of:

a. 100 parts by weight of a rubber mixture containing 20 to 95 weightpercent nitrile rubber and 80 to 5 weight percent of a second rubberselected from the group consisting of natural rubber, neoprene,polybutadiene, styrene-butadiene copolymer and polyisoprene said rubbermixture having an acrylonitrile content in the range of 5 to 35 weightpercent,

b. 1 to 30 parts by weight of a liquid plasticizing agent compatiblewith said rubber mixture and

c. 30 to 60 parts of a reinforcing agent selected from the groupconsisting of hydrated silica, hard clays and carbon black; saidvulcanized mixture characterized as having a volume percent swell after24 hours at 158°F in ASTM number 3 oil in the range of 1 to 40 asmeasured according to ASTM D-471 and a Gehman T₁₀₀ stiffness value at atemperature in the range of -30°F to -100°F as measured according toASTM D-1053.

    __________________________________________________________________________                                     Example Number                               Vulcanized Rubber Composition    1    2    3    4    5    6                   __________________________________________________________________________    I   RUBBER MIXTURE                                                                Nitrile Rubber--Parts by Weight                                                                            60   90   60   90   60   90                      Acrylonitrile, Weight Percent                                                                              18   18   33   33   45   45                      Styrene-Butadiene Copolymer                                                    Parts by Weight             40   10   40   10   40   10                      Weight Percent Acrylonitrile 10.8 16.2 9.8  29.7 27.0 40.5                     Content in Mixture                                                       II  ADDITIVES (PARTS BY WEIGHT)                                                   di(butoxy-ethoxy-ethyl)adipate                                                                             20.0 20.0 20.0 20.0 20.0 20.0                    Carbon Black (HAF)           45.0 45.0 45.0 45.0 45.0 45.0                    Zinc Oxide                   5.0  5.0  5.0  5.0  5.0  5.0                     Stearic Acid                 0.5  0.5  0.5  0.5  0.5  0.5                     Phenyl Beta Naphthylamine    1.0  1.0  1.0  1.0  1.0  1.0                     Benzothiazole Disulfide      1.75 1.75 1.75 1.75 1.75 1.75                    Tetramethyl Thiuram Disulfide                                                                              0.75 0.75 0.75 0.75 0.75 0.75                    Sulfur                       1.5  0.5  1.5  0.5  1.5  0.5                 III PHYSICAL PROPERTIES OF VULCANIZED MIXTURE                                     Gehman T.sub.100 Stiffness, °F (ASTM D-1053)                                                        -65  -59  -62  -44  - 31 17                      Swell, Volume Percent, in ASTM Oil No. 3                                                                   26   15   20   4    14   -2                      After 24 Hours at 158°F (ASTM D-741)                               __________________________________________________________________________

A more preferred embodiment of the conveyor belt composition of thepresent invention comprises a flexible conveyor belt as disclosed abovewherein said vulcanized mixture consists essentially of:

a. 100 parts by weight of a rubber mixture containing 40 to 90 weightpercent nitrile rubber and 60 to 10 weight percent of a second rubberselected from the group consisting of polybutadiene, styrene-butadienecopolymer, neoprene, polyisoprene and natural rubber said mixture havingan acrylonitrile content in the range of 5 to 35 weight percent,

b. 1 to 30 parts by weight of a liquid plasticizing agent selected fromthe group consisting of trioctyl phosphate, dibutyl sebacate, diisooctylsebacate, tributyl citrate, di(butoxy-ethoxy-ethyl) formal,di(butoxy-ethoxy-ethyl)adipate, dioctyl sebacate, dioctyl phthalate,dibutyl phthalate and tributoxy oxyethyl phosphate, and

c. 30 to 60 parts by weight of carbon black, said vulcanized mixturecharacterized as having a volume percent swell after 24 hours in ASTMnumber 3 oil at 158°F in the range of 1 to 40 as measured according toASTM D-471 and a Gehman T₁₀₀ stiffness value at a temperature in therange of -30°F to -100°F as measured according to ASTM D-1053.

The preferred liquid plasticizing agent for use in the flexible conveyorbelt compositions herein disclosed are di(butoxy-ethoxy-ethyl)adipateand di(butoxy-ethoxy-ethyl) formal.

Other desirable physical characteristics of the rubber component of theconveyor belt are:

a. Cure time should be 20 to 40 minutes at 295°F

b. Hardness = Shore "A" should be in the range of 59 to 61 (ASTM D-2240)

c. Abrasion value should be at least 80 according to (ASTM D-2228) and

d. Tensile strength should be at least 2000 psi according to (ASTMD-412).

General procedures for preparing rubber conveyor belts are known to theart. The use of the novel rubber compositions of the present inventionprovides a substantial improvement in this technology.

The present invention also comprises an improved method for transportingtar sands. Specifically, the method of the present invention is animproved method for transporting tar sands in an open pit whereinambient temperatures range between -100°F and +100°F in a manner wherebysticking of the tar sands to the surface of the belt is substantiallyreduced. Specifically this method comprises:

a. providing a continuous flexible rubber surfaced conveyor belt betweena receiving area and a discharge area said rubber surface beingcharacterized as having a volume percent swell after 24 hours in ASTMnumber 3 oil at 158°F in the range of 1 to 40 as measured according toASTM D-471 and a Gehman T₁₀₀ stiffness value at a temperature in therange of -30°F to -100°F according to ASTM D-1053,

b. applying a thin layer of liquid hydrocarbon containing up to 30weight percent aromatics to the upper surface of said belt immediatelybefore contacting said surface with the tar sands to be transportedthereon and thereafter

c. transporting tar sands on the surface of said belt from a receivingarea to a discharge area.

The preferred method of the present invention is a method oftransporting tar sands according to the method above wherein the rubbercomponent of said rubber conveyor belt comprises those mentioned above,preferably a vulcanized mixture of:

a. 100 parts of a rubber mixture comprised of 20 to 95 weight percentnitrile rubber and 80 to 5 weight percent of a solid plasticizerselected from the group consisting of polybutene, styrene-butadienecopolymer, neoprene, polyisoprene and natural rubber said mixture havingan acrylonitrile content in the range of 5 to 35 weight percent;

b. 1 to 30 parts by weight of a liquid plasticizing agent compatiblewith said mixture and

c. 30 to 60 parts by weight of a reinforcing agent selected from thegroup consisting of carbon black, hard clay and hydrated silica.

The liquid plasticizing agent is selected from the group of liquidplasticizing agents compatible with the rubber mixture disclosed above.The preferred reinforcing agent is carbon black.

Although the novel rubber compositions herein provided have beenprimarily disclosed for use in conveyor belts, their application is notlimited to that use. The novel rubber herein disclosed is suitable foruse in making pump diaphragms, pump seals, flexible couplings, gasketsand other equipment requiring low temperature flexibility and swellresistance to liquid hydrocarbons.

The invention claimed is:
 1. A flexible conveyor belt particularlysuited for use in transporting tar sands and having limited resistanceto swelling when exposed to liquid hydrocarbons containing up to 30weight percent aromatics said conveyor comprising a rubber belt having aplurality of metal cables longitudinally disposed in said belt andarranged substantially parallel to each other said belt having a rubbersurface comprising a vulcanized mixture of:a. 100 parts by weight of arubber mixture containing 20 to 95 weight percent nitrile rubber and 80to 5 weight percent of a second rubber selected from the groupconsisting of natural rubber, neoprene, polybutadiene, styrene-butadienecopolymer and polyisoprene said rubber mixture having an acrylonitrilecontent in the range of 5 to 35 weight percent, b. 1 to 30 parts byweight of a liquid plasticizing agent compatible with said rubbermixture and c. 30 to 60 parts of a reinforcing agent selected from hardclay and hydrated silica carbon black, said vulcanized mixturecharacterized as having a volume percent swell after 24 hours at 158°Fin ASTM number 3 oil in the range of 1 to 40 as measured according toASTM D-471 and a Gehman T₁₀₀ stiffness value at a temperature in therange of -30°F to -100°F as measured according to ASTM D-1053.
 2. Aflexible conveyor belt according to claim 1 wherein said vulcanizedmixture consists essentially of:a. 100 parts by weight of a rubbermixture containing 40 to 90 weight percent nitrile rubber and 60 to 10weight percent of a second rubber selected from the group consisting ofpolybutadiene, styrene butadiene copolymer, neoprene, polyisoprene andnatural rubber said mixture having an acrylonitrile content in the rangeof 5 to 35 weight percent, b. 1 to 30 parts by weight of a liquidplasticizing agent selected from the group consisting of trioctylphosphate, dibutyl sebacate, diisooctyl sebacate, tributyl citrate,di(butoxy-ethoxy-ethyl) formal, di(butoxy-ethoxy-ethyl)adipate, dioctylsebacate, dioctyl phthalate, dibutyl phthalate and tributoxy oxyethylphosphate, and c. 30 to 60 parts by weight of carbon black per 100 partsof rubber said vulcanized mixture characterized as having a volumepercent swell after 24 hours at 158°F in ASTM number 3 oil in the rangeof 1 to 40 as measured according to ASTM D-471 and a Gehman T₁₀₀stiffness value at a temperature in the range of -30° F to -100°F asmeasured according to ASTM D-1053.
 3. An improved method fortransporting tar sands in a manner whereby sticking of the tar sands tothe surface of the belt is substantially reduced comprising:a. providinga continuous flexible rubber surfaced conveyor belt according to thecomposition of claim 1 between a receiving area and a discharge areasaid rubber surface being characterized as having a volume percent swellafter 24 hours at 158°F in ASTM number 3 oil in the range of 1 to 40 asmeasured according to ASTM D-471 and a Gehman T₁₀₀ stiffness value at atemperature in the range of -30°F to -100°F according to ASTM D-1053, b.applying a thin layer of liquid hydrocarbon containing a maximum of 30weight percent aromatics to the upper surface of said belt immediatelybefore contacting said surface with the tar sands to be transported andthereafter, c. transporting tar sands on the surface of said belt from areceiving area to a discharge area.
 4. A method according to claim 3wherein said reinforcing agent is carbon black and said liquidplasticizer is selected from the group consisting of trioctyl phosphate,dibutyl sebacate, diisooctyl sebacate, tributyl citrate,di(butoxy-ethoxy-ethyl) formal, di(butoxy-ethoxy-ethyl)adipate, dioctylsebacate, dioctyl phthalate, dibutyl phthalate and tributoxy oxyethylphosphate.